My Semi-Educated Guess on the Black Spot Problem

Started Dec 9, 2008 | Discussions thread
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Cliff Chase
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My Semi-Educated Guess on the Black Spot Problem
Dec 9, 2008

For years I designed very high-speed data acquisition systems. One of the problems in designing these systems is respecting the settling time of multiplexers. Multiplexers take a finite amount of time to settle to some percentage of the previous value on the output. The greater the delta from the previous value the longer it takes to settle.

Another phenomenon with them is that they can "ring", especially if the output is saturated. So if you have a very high value on the output and you then switch the mux to a very low input the output can oscillate slightly. If you don't wait long enough before sampling then you get an error.

Looking at the black dots to me looks exactly like a problem I had with a very large, very fast acquisition system I once designed. Whenever the output was at saturation the following samples would undershoot. I ended up having to use smaller multiplexers so as to isolate the number of switches that were common to reduce the capacitive load on the output.

My guess is that they are reading out too fast and the multiplexers aren't settling before they switch to the next pixel. If this is the case it can't really be fixed in firmware. Also, it could vary between sensors due to process variation.

Then again I could be totally wrong.

CC

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